An MD&DI January 1997 Column
Device manufacturers face a number of challenges when trying to bring their products to market. In addition to designing and manufacturing devices, they must ensure that the devices are adequately sterilized. The sterilization process requires manufacturers to perform such tasks as validation and bioburden studies. Marjorie Lang, a consultant with Lang Consulting, Inc. (Minnetonka, MN), instructs readers to revalidate their sterilization processes when changing the levels of sterilants. She also explains why bioburden studies are necessary for devices to be sterilized with gamma irradiation.
Q. Our firm plans to change its sterilization procedure to use a 10/90 ethylene oxide (EtO)/hydrochlorofluorocarbon (HCFC) cycle instead of a 12/88 EtO/Freon cycle. Do we need to revalidate our process, or can we proceed, using this drop-in gas replacement?
A. Companies that make such a change need to revalidate their sterilization process for microbial lethality, package integrity, EtO residual levels, and, depending on the design of some devices, poststerilization performance characteristics. The reason for this is that, even though HCFC carrier gases were designed to duplicate the chemical inertness and blending characteristics of Freon, EtO/HCFC mixtures contain slightly less available EtO and possess slightly different physical properties in regard to vapor pressures and boiling points. Such differences necessitate the use of lower evacuation levels and higher process pressures than were typically needed in 12/88 cycles in order to obtain the desired EtO concentration. In some cases, a slightly longer gas exposure time may also be needed. Additionally, unless the sterilization chamber has been refitted, chamber fill rates may also be affected because of the slight difference in the amount of heat required for chamber charging.
Q. Why do we have to evaluate product bioburden for devices we intend to sterilize with gamma irradiation? Can't we simply process the devices at a high dose level and assume that the process will result in at least the minimum sterility assurance level?
A. There are two important reasons for matching the level of gamma irradiation to the product bioburden. First, researchers need to determine the amount of product bioburden and demonstrate that it is consistent, so that a baseline number is available to calculate the sterility assurance level conferred by the delivered dose.
Second, gamma irradiation will always cause some change to materials, either immediately or over time, and these changes are best minimized by using as low a dose as possibleeven if the materials initially appear capable of withstanding a high dose. Because odd radioresistant organisms do exist, and bioburden levels can spike or drift out of control, sterilizers need to audit doses quarterly to ensure that the minimum sterility assurance level is being achieved.
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